DARIEN, Conn. — Make sure you don’t have radon in your house by picking up a free radon test kit at the Darien Health Department this month. January is designated as National Radon Action Month by the federal Environmental Protection Agency.

Radon is an invisible, odorless and tasteless radioactive gas formed from the decay of radium and uranium, which occur naturally in bedrock worldwide. Higher concentrations are possible near certain types of bedrock such as granite and shale, which have higher uranium content. Test data suggests that such higher concentrations occur sporadically in all parts of the state.

Exposure to elevated radon levels in indoor air is a leading cause of lung cancer in non-smokers, according to the surgeon general. Exposure does not cause any immediately noticeable symptoms. Long-term exposure to high levels could increase a person’s risk, with adverse affects taking up to 30 years to develop.

Testing should be done in the winter, when radon levels are commonly higher. Test kits are available in the Health Department offices at Town Hall, 2 Renshaw Road, from 8:30 a.m. to 4:30 p.m. weekdays. Instructions and fact sheets accompany the kits, which are provided with an accompanying postage-free mail-in envelope.